The invention relates generally to random access channels in a wireless code division multiple access communication system. More specifically, the invention relates to raising the packet data payload of packets transferred over a random access channel.
FIG. 1 depicts a wireless spread spectrum Code Division Multiple Access (CDMA) communication system 20. A base station 22 communicates with user equipment units (UEs) 241-24n in its operating area. In a spread spectrum CDMA system 18, data signals are communicated between UEs 241-24n and the base station 22 over the same spectrum. Each data signal in the shared spectrum is spread with a unique chip code sequence. Upon reception, using a replica of the chip code sequence, a particulardata signal is recovered.
Since signals are distinguished by their chip code sequences (code), separate dedicated communication channels are created using different codes. Signals from the base station 22 to the UEs 241-24n are sent on downlink channels and signals from the UEs 241-24n to the base station 22 are sent on uplink channels. For coherent detection of downlink transmissions by the UEs 241-24n, pilot signals are transmitted to all the UEs 241-24n within the base station's operating range. The UEs 241-24n condition their receivers based on the pilot signals to enable data reception.
In many CDMA systems 20, random access channels are used by multiple users. One random access channel is a common packet channel (CPCH) which is used for uplink transmissions. A CPCH is capable of carrying packets of data from different UEs 241-24n. Each UE's packets have an associated uplink scrambling code which distinguishes it from other UE's packets. The CPCH is typically used to carry infrequently communicated data at high rates.
The CPCH as shown in FIG. 2 is time divided into frames 301-30m having time slots 281-28n, such as eight time slots proposed for the Third Generation Mobile Telecommunications System (IMT-2000)-UMTS. The UE 241 transmits a data packet over the CPCH using an assigned uplink scrambling code starting in a specific time slot. The packet typically lasts for multiple frames 301-30m. The uplink packets are used to carry data and control signals. The base station 22 receives and recovers data from the uplink data packet. Typically, a dedicated downlink control channel is established for use in sending control signals from the base station 22 to the UE 241 as well as other data.
FIG. 3 is a simplified UE 241 and base station 22 for transmitting and receiving packet data over a random access channel 42 in a CDMA system 20. A data packet is generated by a data packet generator 32 at the UE 241. The data is subsequently encoded for error protection, such as by convolutional encoding, by an encoder 34. The encoded data is spread by a spreading device 36, such as by mixing the encoded data with a spreading code. The spread data is modulated to radio frequency by a modulator 38 and radiated by the UE's antenna 40.
The radio frequency signal is sent through a random access channel 42, such as the CPCH, to the base station 22. An antenna 44 at the base station 22 receives radio signals. The radio signals are demodulated by a demodulator 46 to produce a baseband signal. The baseband signal is despread by a despreading device 48, such as by mixing the baseband signal with a replica of the original spreading code. The de-spread signal is passed through a decoder 50 to recover the original data packet.
CDMA systems 20 typically use some form of adaptive power control. In a CDMA system 20, many signals share the same spectrum. When a UE 241 or base station 22 receives a specific signal, all the other signals within the same spectrum are noise-like in relation to the specific signal. Increasing the power level of one signal degrades all the other signals within the same spectrum. However, reducing transmission power levels too far results in undesirable received signal quality at the receivers. To maintain a desired signal quality at the minimum transmission power level, transmission power control is used.
Along with other data, both the UE 241 and base station 22 send transmission power control (TPC) signals. The TPC signals tell the receiving UE 241 or base station 22 to increase or decrease its transmission power level based on the quality of the received signals. Once the UE 241 or base station 22 receives the TPC signals, it adjusts its transmission power level accordingly. For signals sent from the UE 241 to the base station 22 on the CPCH, typically, a closed loop power control preamble (CLPC-Preamble) is used to carry the TPC signals. The CLPC-Preamble precedes other data being carried in the packet.
To assure that the initial power level of uplink communications is not set too high, the UE 24 initially sets its transmission power level at a sufficiently low power level. The initial power level is set such that it is highly unlikely that the transmitted data will cause a degradation to reception of other UEs. Both the UE 241 and base station 22 have their transmission power levels adjusted to a desired transmission power level by the corresponding uplink and downlink TPC signals.
Until either the UE 241 or base station 22 reaches a sufficient transmission power level, no data is being transferred. This lack of data transfer is an inefficient use of the systems resources and is undesirable. Accordingly, it is desirable to increase data transfer in such systems.